1. Field of the Invention
The present invention relates to a focusing control apparatus and method for an optical pickup device which focuses a light beam to be incident on a multi-layer recording medium having a plurality of recording layers and corrects aberration of the light beam reflected from the multi-layer recording medium.
2. Description of the Related Art
A DVD (digital versatile disc) is an optical recording medium which was developed for utilization in recording and reproducing digital data such as computer application programs and data. For example, the DVD having a diameter of approximately 12 centimeters (cm) has a data recording capacity as much as approximately seven times a CD (compact disc) on a single side of the disc. In order to achieve a larger capacity and a higher density of optical disc, it is effective to reduce the wavelength of a light source and increase the numerical aperture (NA) of an objective lens. On the other hand, a multi-layer optical disc having a plurality of recording layers is now under development as an approach for increasing the recording capacity of an optical disc.
It is well known that spherical aberration occurs, during recording or reproduction, due to variations in thickness of a transparent cover layer formed on a recording layer through which a light beam is transmitted. The spherical aberration brings about degradation in recorded or reproduced signal. The amount of spherical aberration occurring with respect to the thickness error of the transparent cover layer is expressed by the following equation, and is disclosed, for example, in J. Bratt, Applied Optics, Vol. 36, No. 32, p 8459 (1997). Particularly, when the numerical aperture of an objective lens is 0.6 or more, higher order aberration cannot be neglected, and up to eighth-order aberration must be taken into consideration.
W80=xe2x88x925(n6xe2x88x921)NA8xc2x7xcex94T/128n7xc2x7xcex
where n is the refractive index of the cover layer; NA is the numerical aperture of the objective lens; xcex94T is the thickness error of the cover layer; and xcex is the wavelength of the light source.
Thus, there is the following problem in a multi-layer optical disc recording/reproducing apparatus which uses an objective lens having a large numerical aperture, particularly, an objective lens having a numerical aperture of 0.8 or more. When a multi-layer disc is recorded or reproduced, a laser light beam is focused on any one of the recording layers, and a signal is recorded on the recording layer or a signal is reproduced from the recording layer while aberration correction is performed. During recording or reproduction a focused position of the irradiated light beam may be changed from one recording layer to another. The movement of the focused position of the irradiation light beam in this manner is generally referred to as xe2x80x9cfocus jumpxe2x80x9d. When the focus jump is performed, large spherical aberration occurs on the other recording layer, to which the focus is jumped, if an objective lens of a large numerical aperture is used, as is apparent from the aforementioned equation for the amount of aberration. This spherical aberration not only degrades the quality of a recorded or reproduced data signal but also degrades a signal associated with a drive system such as focusing, tracking and so on.
In a conventional algorithm for focusing and aberration correction, a focus pull-in operation is performed onto a recording layer intended for recording or reproduction. Then, acquisition of control information such as focus servo, tracking servo, addresses,and the like is performed. Subsequently, after the spherical aberration is corrected to provide the best address signal and reproduced signal, recording or reproduction is conducted. FIG. 1 shows the intensity of a focus error signal when the focus is jumped to a second recording layer, while the spherical aberration has been corrected for a first recording layer, with respect to the depth from the surface of a disc. As the focused position is moved in the depth direction, the focus error signal exhibits an xe2x80x9cS-shapedxe2x80x9d characteristic or sigmoid curve with respected to a focus shift. In other words, the focus error signal is generated in the shape of S centered at a focused position (hereinafter, referred to as an S-shaped waveform). In this event, the focusing position of the objective lens is moved while a focus servo loop is open. As illustrated, the S-shaped waveform corresponding to the second recording layer exhibits a small magnitude, and a collapsed waveform is detected. With such an S-shaped waveform, it is difficult to pull-in the focus onto the second recording layer, possibly resulting in a failed focus jump.
The present invention has been made in view of the foregoing problem, and it is an object of the invention to provide a focusing control apparatus and method which is capable of performing a reliable focus control for an optical pickup device which has a high NA (numerical aperture) objective lens and an aberration correction capability.
To achieve the object, according to one aspect of the present intention, there is provided a focusing control apparatus for an optical pickup which includes an objective lens for focusing a light beam on a recording layer of a multi-layer recording medium having a plurality of recording layers, an aberration correcting unit for correcting aberration of a reflected light beam from the recording layer, and a photodetector for receiving the reflected light beam, the focusing control apparatus comprises a focus error detector which detects an amount of focus error of the light beam from a detection signal of the photodetector to generate a focus error value; an aberration detector which detects an amount of aberration of the reflected light beam from a detection signal of the photodetector to generate an aberration value; and a controller which controls a focused position of the objective lens on the basis of the focus error value while adjusting an amount of aberration correction of the aberration correcting unit on the basis of the aberration value, wherein the controller performs a focus jump from one recording layer to another recording layer after adjusting the amount of aberration correction of the aberration correcting unit for the another recording layer.
According to another aspect of the present invention, there is provided a focusing control apparatus for an optical pickup which includes an objective lens for focusing a light beam on a recording layer of a multi-layer recording medium having a plurality of recording layers, an aberration correcting unit for correcting aberration of a reflected light beam from the recording layer, and a photodetector for receiving the reflected light beam, the focusing control apparatus comprises a focus error detector which detects an amount of focus error of the light beam from a detection signal of the photodetector to generate a focus error value; an aberration detector which detects an amount of aberration of the reflected light beam from a detection signal of the photodetector to generate an aberration value; a controller which controls a focused position of the objective lens on the basis of the focus error value while adjusting an amount of aberration correction of the aberration correcting unit on the basis of the aberration value, a calculator which calculates an interlayer distance between one recording layer and another recording layer from a change in the focus error value when a focused position of the objective lens is changed from the one recording layer to the another recording layer; and an estimator which calculates an estimated aberration correction value for the aberration correcting unit when the light beam is focused on the another recording layer using the interlayer distance and an amount of aberration correction of the aberration correcting unit when the light beam is focused on the one recording layer, wherein the controller performs a focus jump from the one recording layer to the another recording layer after adjusting the amount of aberration correction of the aberration correcting unit using the estimated aberration correction value.
According to another aspect of the present invention, there is provided a focusing control apparatus for an optical pickup which includes an objective lens for focusing a light beam on a recording layer of a multi-layer recording medium having a plurality of recording layers, an aberration correcting unit for correcting aberration of a reflected light beam from the recording layer, and a photodetector for receiving the reflected light beam, the focusing control apparatus comprises a focus error detector which detects an amount of focus error of the light beam from a detection signal of the photodetector to generate a focus error value; an aberration detector which detects an amount of aberration of the reflected light beam from a detection signal of the photodetector to generate an aberration value; a controller which controls a focused position of the objective lens on the basis of the focus error value while adjusting an amount of aberration correction of the aberration correcting unit on the basis of the aberration value, and a memory which stores an amount of aberration correction when the light beam is focused on each of the plurality of recording layers, wherein the controller performs a focus jump from one recording layer to another recording layer after adjusting the aberration correcting unit with the amount of aberration correction for the another recording layer stored in the memory.
According to another aspect of the present invention, there is provided a focusing control method for an optical pickup which includes an objective lens for focusing a light beam on a recording layer of a multi-layer recording medium having a plurality of recording layers, an aberration correcting unit for correcting aberration of a reflected light beam from the recording layer, and a photodetector for receiving the reflected light beam, the method comprises the steps of detecting an amount of focus error of the light beam from a detection signal of the photodetector to generate a focus error value; detecting an amount of aberration of the reflected light beam from a detection signal of the photodetector to generate an aberration value; controlling a focused position of the objective lens on the basis of the focus error value while adjusting an amount of aberration correction of the aberration correcting unit on the basis of the aberration value; and performing a focus jump from one recording layer to another recording layer after adjusting the amount of aberration correction of the aberration correcting unit for the another recording layer.
According to another aspect of the present invention, there is provided a focusing control method for an optical pickup which includes an objective lens for focusing a light beam on a recording layer of a multi-layer recording medium having a plurality of recording layers, an aberration correcting unit for correcting aberration of a reflected light beam from the recording layer, and a photodetector for receiving the reflected light beam, the method comprises the steps of detecting an amount of focus error of the light beam from a detection signal of the photodetector to generate a focus error value; detecting an amount of aberration of the reflected light beam from a detection signal of the photodetector to generate an aberration value; controlling a focused position of the objective lens on the basis of the focus error value while adjusting an amount of aberration correction of the aberration correcting unit on the basis of the aberration value, calculating an interlayer distance between one recording layer and another recording layer from a change in the focus error value when a focused position of the objective lens is changed from the one recording layer to the another recording layer; calculating an estimated aberration correction value for the aberration correcting unit when the light beam is focused on the another recording layer using the interlayer distance and an amount of aberration correction of the aberration correcting unit when the light beam is focused on the one recording layer, and performing a focus jump from the one recording layer to the another recording layer after adjusting the amount of aberration correction of the aberration correcting unit using the estimated aberration correction value.
According to another aspect of the present invention, there is provided a focusing control apparatus for an optical pickup which includes an objective lens for focusing a light beam on a recording layer of a multi-layer recording medium having a plurality of recording layers, an aberration correcting unit for correcting aberration of a reflected light beam from the recording layer, and a photodetector for receiving the reflected light beam, the focusing control apparatus comprises a focus error detector which detects an amount of focus error of the light beam from a detection signal of the photodetector to generate a focus error value; an aberration detector which detects an amount of aberration of the reflected light beam from a detection signal of the photodetector to generate an aberration value; a controller which controls a focused position of the objective lens on the basis of the focus error value while adjusting an amount of aberration correction of the aberration correcting unit on the basis of the aberration value, a calculator which calculates an interlayer distance between one recording layer and another recording layer from a change in the focus error value when a focused position of the objective lens is changed from the one recording layer to the another recording layer; and an estimator which calculates an estimated aberration correction value for the aberration correcting unit when the light beam is focused on the another recording layer using the interlayer distance and an aberration correction value of the aberration correcting unit when the light beam is focused on the one recording layer, wherein the controller performs a focus jump from the one recording layer to the another recording layer after adjusting the amount of aberration correction of the aberration correcting unit to be an intermediate value between the aberration correction value for the one recording layer and the estimated aberration correction value for the another recording layer.
According to another aspect of the present invention, there is provided a focusing control method for an optical pickup which includes an objective lens for focusing a light beam on a recording layer of a multi-layer recording medium having a plurality of recording layers, an aberration correcting unit for correcting aberration of a reflected light beam from the recording layer, and a photodetector for receiving the reflected light beam, the method comprises the steps of detecting an amount of focus error of the light beam from a detection signal of the photodetector to generate a focus error value; detecting an amount of aberration of the reflected light beam from a detection signal of the photodetector to generate an aberration value; controlling a focused position of the objective lens on the basis of the focus error value while adjusting an amount of aberration correction of the aberration correcting unit on the basis of the aberration value, calculating an interlayer distance between one recording layer and another recording layer from a change in the focus error value when a focused position of the objective lens is changed from the one recording layer to the another recording layer; calculating an estimated aberration correction value for the aberration correcting unit when the light beam is focused on the another recording layer using the interlayer distance and an aberration correction value of the aberration correcting unit when the light beam is focused on the one recording layer, performing a focus jump from the one recording layer to the another recording layer after adjusting the amount of aberration correction of the aberration correcting unit to be an intermediate value between the aberration correction value for the one recording layer and the estimated aberration correction value for the another recording layer.